The present invention relates to a non cut-off, Class B, emitter-follower, single-ended push-pull (SEPP) circuit.
An emitter-follower type SEPP circuit is generally operated in Class B from a point of view of efficiency. In such a circuit, it is essential to pass an idle current through the output transistors in order to smoothly interconnect the upper and lower (positive and negative half cycles) transfer characteristics, that is, to reduce crossover distortion. In an ordinary circuit of this type, crossover distortion may occur since one of transistors will be turned on when the other of the transistors is cut off. To eliminate this problem, recently a non cut-off Class B circuit has been used in which such cut-off is not permitted due to the provision of a constant idle current which always flow in the otherwise nonconducting transistor. A servo circuit is used to generate this idle current. With this construction, crossover distortion is indeed reduced. However, this approach does nothing regarding current distortion arising from the non-linearity of the current transfer characteristic and voltage distortion due to the exponential transfer characteristics of the transistor.
Further, there is a disadvantage in that thermal runaway can occur if the idle current is not fully compensated for. In the prior art arrangement, the idle current alters little with changes in signal levels and ambient temperature. Thus, the circuit's operating point can shift either over long or short times.
Further, it is extremely difficult to design such a circuit since the above-mentioned thermal compensation is very sensitive. Particularly, the design is made difficult in a conventional non cut-off Class B circuit which does not utilize feedback. In any event, temperature compensation cannot be complete in any such circuit design.
FIG. 1 shows the basic construction of a conventional non cut-off Class B SEPP circuit. In this figure, symbols A.sub.1 and A.sub.2 designate error amplifiers having a gain less than one, B.sub.1 and B.sub.2 voltage generating circuits which may be voltage adders, C an input signal source, and V.sub.B a bias voltage for bipolar transistors Q.sub.1 and Q.sub.2.
In FIG. 1, an idle current I.sub.d is composed of currents i.sub.E1 and i.sub.E2, which are present when no signal is applied to an input terminal IN. Currents I.sub.B1 and I.sub.B2 are supplied from a power source V.sub.B. With the base-emitter voltage of each transistor being V.sub.BE and the emitter resistance thereof R.sub.E, the following equation can be written: ##EQU1## When an input signal current i.sub.i flows, the current i.sub.E1 is increased. Assuming that the current amplification factor of the transistor Q.sub.1 is h.sub.fe1, the following equation is obtained: EQU i.sub.E1 =h.sub.fe1 .multidot.i.sub.i.
This current i.sub.E1 produces a voltage across the resistor R.sub.E. The input voltage V.sub.i1 to the amplifier A.sub.1 then becomes: EQU V.sub.i1 =(V.sub.BE -V.sub.B)+i.sub.E1 R.sub.E =(V.sub.BE -V.sub.B)+h.sub.fe1 i.sub.i R.sub.E.
The presence of this voltage causes the base of the transistor Q.sub.2 to be biased reversely and would cut-off the transistor if the amplifier A.sub.1 were not provided. If the gain of the amplifier A.sub.1 is set to one, the voltage V.sub.i1 is positively fed back to the base of the transistor Q.sub.1 as it is, raising the input voltage to the transistor Q.sub.1. Thus, a constant idle current (I.sub.d) will always be provided without reversely biasing the transistor Q.sub.2. A quite similar operation is carried out if the input current i.sub.i is reversed to flow in the direction in which the transistor Q.sub.2 is turned on.
FIG. 2 shows a current transfer characteristic with respect to the input signal current i.sub.i in the circuit of FIG. 1. Generally, as the emitter current of a transistor increases at high current levels, h.sub.fe (current amplification factor) decreases, and hence the resultant characteristic is considerably non-linear, resulting in a large amount of current distortion. Moreover, if the gains of the amplifiers A.sub.1 and A.sub.2 are set to one as described above, the positive feedback ratio will be 100% and the stabilizing effect of the resistor R.sub.E will be completely lost (R.sub.E becomes zero in equation (1)). Therefore, the idle current I.sub.d becomes unstable and oscillations may occur. Practically, the gains of the transistors are set to values less than one, but the circuit is still quite unstable with respect to temperature.
If a non cut-off Class B circuit configuration is not employed, and instead constant voltage driving is effected by dispensing with amplifiers A.sub.1 and A.sub.2 in FIG. 1 in an attempt to reduce the distortion due to the nonlinear current transfer characteristic, the transfer characteristic will be as shown in FIG. 3. Even in this case, however, distortion due to the exponential transfer characteristics of transistors remains.
Thus, in a conventional Class B SEPP circuit employing constant voltage driving, (1) crossover distortion due to the exponential transfer characteristic and distortion due to the on-off operations of the output transistors occurs. Even if a non cut-off Class B SEPP circuit using constant current driving is used, (2) distortion due to a nonlinear current transfer characteristic occurs. Further, irrespective of the driving method, (3) temperature compensation for the idle current is necessary, but complete compensation is impossible. Also, (4) times longer than several tens of minutes are needed until the idle current becomes constant after power is applied. Still further, (5) the idle current fluctuates with the presence and absence of the input signal, and the magnitude of the idle current deviates greatly from a set value when a large signal is present. Finally, (6) the operating point becomes unstable and varies with ambient temperature and the presence and absence of the input signal due to the above-mentioned disadvantages (3) to (5).
To obviate the above-mentioned disadvantages, it is an object of the present invention to provide a non cut-off, Class B, emitter-follower SEPP circuit which has extremely small distortion and in which it is completely unnecessary to temperature compensate the idle current.
Further, the adjustment of the idle current setting of an output amplifying element in an SEPP type amplifier has heretofore been carried out mainly by manually adjusting a variable resistor. Because the idle current setting circuit includes a thermal compensating element such as a varistor or a thermistor, the idle current will vary with time and temperature. Typically, several to several tens of minutes are needed for reaching a constant idle current state after power has been applied. Further, there is a disadvantage in that so-called thermal distortion (thermal cross-modulation distortion) can result from fluctuations of the operating point due to level changes of an input signal. Even if a push-pull arrangement in which the transistors are operated in Class A is employed, odd-order harmonics of the input signal are amplified (although even-order harmonics are suppressed to a certain degree).
In view of the foregoing, another object of the present invention is to provide a bias control circuit for an amplifier capable of stabilizing a transistor amplifier circuit by maintaining the DC idle current of an amplifying element substantially constant independent of temperature, and which is thus capable of reducing crossover distortion, switching distortion and the like.